Asphalt-paved road surfaces are subject to deterioration caused by a number of factors. For example, seasonal temperature fluctuations cause the road surface to become brittle and/or cracked. Erosion of the road bed beneath the road surface may also result in cracking. Moreover, certain of the chemical constituents incorporated in fresh asphalt are gradually lost or their properties change with time, further contributing to brittleness and/or cracking of the road surface.
The prior art has evolved a variety of techniques for rejuvenating asphalt-paved road surfaces. Most prior art processes involve the use of large banks of heaters which heat the road surface to a temperature in the 240.degree.-350.degree. F. range to a depth of about 1-2 inches. Such heating softens the upper, exposed layer of asphalt which can then be broken up with conventional mechanical grinders or scarifiers. Typically, a small amount of fresh asphalt or asphalt rejuvenant is mixed with the heated, broken asphalt and this mixture is then distributed over the road surface and pressed back into position to leave a smooth, finished road surface. In some cases, the heated, broken material is removed altogether from the road surface, mixed with fresh asphalt or asphalt rejuvenant material and then returned to the road surface and pressed back into position.
The prior art is exemplified by U.S. Pat. No. 3,807,886 issued 30 Apr., 1974 for an invention of Earl F. Cutler entitled "Method for Heating Asphalt Concrete Roadways and the Like"; U.S. Pat. No. 3,843,274 issued 22 Oct., 1974 for an invention of Nathan Gutman et al entitled "Asphalt Reclaimer"; U.S. Pat. No. 3,874,366 issued 1 Apr., 1975 for an invention of Earl F. Cutler entitled "Exhaust Manifold for Asphalt Concrete Heating Apparatus"; U.S. Pat. No. 3,970,404 issued 20 Jul., 1976 for an invention of Angelo W. Benedetti entitled "Method of Reconstructing Asphalt Pavement"; U.S. Pat. No. 3,997,276 issued 14 Dec., 1976 for an invention of James A. Jackson, Sr. entitled "Road Maintenance Machine and Methods"; U.S. Pat. No. 4,129,398 issued 12 Dec., 1978 for an invention of Walter Schoelkopf entitled "Method and Apparatus for Plastifying and Tearing Up of Damaged Road Surfaces and Covers"; U.S. Pat. No. 4,335,975 issued 22 Jun., 1982 for a further invention of Walter Schoelkopf having the same title; U.S. Pat. No. 4,226,552 issued 7 Oct., 1980 for an invention of Frank F. Moench entitled "Asphaltic Pavement Treating Apparatus and Method"; and, U.S. Pat. No. 4,534,674 issued 13 Aug., 1985 for an invention of Earl F. Cutler entitled "Dual Lift Repaving Machine".
The inventors consider the prior art techniques aforesaid to be undesirable because they are incapable of heating the asphalt road surface to an adequate depth without risking irreversible damage to the asphalt material through application of excessive heat, or without extending the time required for the process to the point that it becomes uneconomical. More particularly, the inventors believe that in order to adequately rejuvenate an asphalt-paved road surface, at least 2 inches of asphalt material must be removed from the road surface, mixed with fresh asphalt or an asphalt rejuvenant agent (depending upon the extent to which the existing asphalt has deteriorated), and then returned to the road surface (this of course assumes that the existing road surface has not deteriorated so badly that it must be replaced in its entirety).
Most roads are paved with asphalt to a minimum depth of about 3 inches. If the road surface is cracked, then asphalt must be removed to a depth of at least 2/3 of the crack penetration depth or else the crack will quickly reappear after the road surface is rejuvenated in accordance with prior art techniques like those described above. Unfortunately however, these state-of-the-art processes permit asphalt to be removed from the road surface to a depth of only about 1 inch. It can thus be seen that if a 3 inch road surface has a crack which extends completely through the road surface then it will not be possible, with the prior art techniques, to remove material from the road surface to a depth of 2/3 of the crack penetration depth (i.e. 2 inches) and so the crack will likely quickly reappear if the road is resurfaced in conventional fashion.
It takes approximately 4 times as long to heat an asphalt road surface to a depth of about 2 inches as is required to heat the same road surface to the same temperature and to a depth of about 1 inch. It can thus be seen that it would take approximately 4 times as long for prior art techniques like those mentioned above to heat an asphalt road surface to the 2 inch depth preferred by the inventors in order to facilitate removal of asphalt from the road surface to a depth of 2 inches. Moreover, in order to achieve a 2 inch heat penetration depth by conventional methods, excessive heat would have to be applied to the exposed upper asphalt surface. Preferably, asphalt should not be heated to a temperature above about 300.degree. F.-325.degree. F. or else the asphalt's constituent oils may be burned off, resulting in brittleness of the asphalt material which can in turn lead to severe cracking problems, especially during the winter season.
The inventors have devised a technique in which an asphalt-paved road surface is rejuvenated in two stages. The inventors' technique facilitates heating of the asphalt to the desired 2 inch depth without risking damage of the asphalt material through application of excessive heat thereto and also without significantly extending the time required to complete the road resurfacing process.